Nuclear magnetic resonance (NMR) is usually carried out in the highly homogeneous fields generated by large and static superconducting magnets. In general, the samples under study have to be taken into the magnet, limiting the use of NMR for in situ measurements. To overcome these disadvantages portable magnets made from permanent magnetic materials have been proposed as e. g. in DE 199 39 626 C2.
Although in theory, this magnet geometry should provide a quite homogeneous field, in practice, the inhomogeneity of the magnetic material, and the inaccuracy in the size and positioning of the pieces in the final array, lead to important field distortions. These imperfections preclude the use of mobile sensors for high resolution spectroscopy purposes.